The study of extreme environments and the microorganisms that inhabit these environments, the so-called extremophiles, has become increasingly popular in recent years. One important class of extreme environments is those of low pH, which are inhabited by prokaryotic and eukaryotic microorganisms referred to as acidophiles. The ability of microbes to grow at low pH is a seemingly ancient trait, as acidophiles are widely distributed throughout the two prokaryotic domains. Heterotrophic acidophiles can be enriched for, and cultivated in, liquid media containing a variety of single or complex carbon sources. Iron-oxidizing acidophiles were particularly problematic, with some (e.g., Leptospirillum ferrooxidans) being categorized as being incapable of growing on solid media. Recent advances in this area have led to the development of techniques that allow all categorized species of acidophilic prokaryotes to be grown on solid media. Most probable number (MPN) microbial counts of cultures in specified liquid media (e.g., acidic ferrous sulfate medium) and incubated at an appropriate temperature continue to be used to enumerate acidophiles on a physiological basis. More recently, 16S rRNA gene libraries have been prepared from DNA samples obtained at an abandoned pyrite mine at the Iron Mountain site and acidic geothermal sites on the volcanic island of Montserrat.

Distribution of acidophiles among the three kingdoms of living organisms. Those that contain acidophilic organisms are highlighted in bold. Reprinted from Advances in Applied Microbiology (29) with permission of the publisher.

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FIGURE 1

Distribution of acidophiles among the three kingdoms of living organisms. Those that contain acidophilic organisms are highlighted in bold. Reprinted from Advances in Applied Microbiology (29) with permission of the publisher.

Roles of acidophiles in the oxidative dissolution of metal sulfides. The iron-oxidizing prokaryotes (FOP) that are attached to the mineral surface or are free-swimming regenerate ferric iron (indicated by broken arrows). The thiosulfate can be oxidized to sulfuric acid by sulfur-oxidizing prokaryotes (SOP) either directly or following oxidation by ferric iron to polythionates or sulfur. Carbon dioxide fixed and excreted as dissolved organic carbon (DOC) by the autotrophic FOP or SOP can be used to drive iron oxidation by heterotrophic iron-oxidizing acidophiles (HFOP). Heterotrophs contribute to metal sulfide oxidation by removing toxic dissolved organic carbon, oxidizing it to CO2 that the autotrophic FOP/SOP can use. Reprinted from Mine Water and the Environment (31) with permission of the publisher.

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FIGURE 2

Roles of acidophiles in the oxidative dissolution of metal sulfides. The iron-oxidizing prokaryotes (FOP) that are attached to the mineral surface or are free-swimming regenerate ferric iron (indicated by broken arrows). The thiosulfate can be oxidized to sulfuric acid by sulfur-oxidizing prokaryotes (SOP) either directly or following oxidation by ferric iron to polythionates or sulfur. Carbon dioxide fixed and excreted as dissolved organic carbon (DOC) by the autotrophic FOP or SOP can be used to drive iron oxidation by heterotrophic iron-oxidizing acidophiles (HFOP). Heterotrophs contribute to metal sulfide oxidation by removing toxic dissolved organic carbon, oxidizing it to CO2 that the autotrophic FOP/SOP can use. Reprinted from Mine Water and the Environment (31) with permission of the publisher.

55. Peccia,J.,, E.A. Marchand,, J.Silverstein, and, M.Hernandez.2000.Development and application of small-subunit rRNA probes for assessment of selected Thiobacillus species and members of the genus Acidiphilium.Appl. Environ. Microbiol.66:3065– 3072.